Set file_url_prefix trait for opening notebooks directly

Author: afshin

jupyterlab_server/app.py

@@ -21,6 +21,7 @@ class LabServerApp(ExtensionAppJinjaMixin, LabConfig, ExtensionApp):
     name = "jupyterlab_server"
     extension_url = "/lab"
     app_name = "JupyterLab Server Application"
+    file_url_prefix = "/lab/tree"
 
     @property
     def app_namespace(self):
@@ -69,9 +70,9 @@ def app_namespace(self):
         "whitelist_uris": ("allowed_extensions_uris", "1.2"),
     }
 
-    # Method copied from 
+    # Method copied from
     # https://github.com/jupyterhub/jupyterhub/blob/d1a85e53dccfc7b1dd81b0c1985d158cc6b61820/jupyterhub/auth.py#L143-L161
-    @observe(*list(_deprecated_aliases))    
+    @observe(*list(_deprecated_aliases))
     def _deprecated_trait(self, change):
         """observer for deprecated traits"""
         old_attr = change.name

setup.py

@@ -58,7 +58,7 @@
         'jsonschema>=3.0.1',
         'packaging',
         'requests',
-        'jupyter_server~=1.1',
+        'jupyter_server~=1.4',
     ]
 
 if __name__ == '__main__':